10_UTRAN-RRM_ws11
Transcript of 10_UTRAN-RRM_ws11
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UTRAN Radio Resource Management
Introduction
Handover Control
Soft/Softer Handover
Inter Frequency Handover
Power Control
Closed Loop Power Control Open Loop Power Control
Interference Management
Load Control
Call Admission Control
Congestion Control
Packet Data Transmission
Packet Data Control
Dynamic Scheduling
BTS 1
BTS 3
BTS 2UE
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UMTS Networks 2Andreas Mitschele-Thiel, Jens Mueckenheim Nov. 2011
References
H. Holma, A. Toskala (Ed.), WCDMA for UMTS, Wiley, 5th edition, Wiley, 2010
Walke, Althoff, Seidenberg: UMTS Ein Kurs. J. Schlembach Fachverlag, 2002
H. Kaaranen, et.al., UMTS Networks: Architecture, Mobility and Services,Wiley, 2001. (see chapter 4)
A. Viterbi: CDMA: Principles of Spread Spectrum Communications, AddisonWesley, 1995.
J. Laiho, A. Wacker, T. Novosad (ed.): Radio Network Planning andOptimisation for UMTS, Wiley, 2001
T. Ojanper, R. Prasad, Wideband CDMA for Third Generation MobileCommunication, Artech House, 1998.
R. Prasad, W. Mohr, W. Konhuser, Third Generation Mobile Communications
Systems, Artech House, March 2000.
3GPP standards:
TS 25.214: Physical Layer Procedures (esp. power control)
TR 25.922: Radio Resource Management Strategies
TR 25.942: RF System Scenarios
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Efficient use of limited radio resources (spectrum, power, code space)
Minimizing interference Flexibility regarding services (Quality of Service, user behaviour)
Simple algorithms requiring small signalling overhead only
Stability and overload protection
Self adaptive in varying environments
Allow interoperability in multi-vendor environments
Radio Resource Managementalgorithms control theefficient use of resourceswith respect tointerdependent objectives:
cell coverage cell capacity quality of service
RRM High-Level Requirements
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RRM Components
Radio Resource Management
MediumAccessControl
Packet DataControl
LoadControl
HandoverControl
PowerControl
Core Network/ other RNCs
Physical layer
typically inRNC
typically inNodeB
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Handover Control: Basics
General: mechanism of changing a cell or base station during a call or session
Handover in UMTS:
UE may have active radio links to more than one Node B
Mobile-assisted & network-based handover in UMTS:
UE reports measurements to UTRAN if reporting criteria (which are set bythe UTRAN) are met
UTRAN then decides to dynamically add or delete radio links dependingon the measurement results
Types of Handover:
Soft/Softer Handover (dedicated channels)
Hard Handover (shared channels)
Inter Frequency (Hard) Handover
Inter System Handover (e.g. UMTS-GSM)
Cell selection/re-selection (inactive or idle)
All handover types require heavy support from the UMTS network infrastructure!
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Macro Diversity & Soft Handover (Wrap-Up)
Downlink: combining in the mobile stationUplink: combining in the base station and/or radio network controller
NodeB 1NodeB 2
UE
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Soft/ Softer Handover
In soft/softer handover the UE maintains active radio links to more than oneNode B
Combination of the signals from multiple active radio links is necessary
Soft Handover
The mobile is connected to (at least) two cells belonging to different NodeBs
In uplink, the signals are combined in the RNC,e.g. by means ofselection combining using CRC
Softer Handover
The mobile is connected to two sectors within one NodeB More efficient combining in the uplink is possible like
maximum ratio combining (MRC) in the NodeB instead of RNC
Note:
In uplink no additional signal is transmitted, while in downlink each new linkcauses interference to other users, therefore:
Uplink: HO general increase performance
Downlink: Trade-off
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Soft and Softer Handover in Practice
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Soft Handover Control
Measurement quantity, e.g.EC/I0 on CPICH
Relative thresholds add &drop for adding & dropping
Preservation time Tlink toavoid ping-pong effects
Event triggered measurementreporting to decreasesignalling load
MeasurementQuantity
Link to 1 Link to 1 & 2 Link to 2 time
CPICH 1
CPICH 2
drop
NodeB 1 NodeB 2
add Tlink
UE
soft handoverarea
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Soft Handover Simulation Results
Soft handover significantly improves the performance, but
0%
5%
10%
15%
20%
25%
5 15 25 35 45 55
Offered Traffic [Erlang per site]
Ou
tage
Pro
ba
bility
(Bloc
king
an
dDropp
ing
)
1 link
max 2 SHO links
max 4 SHO links
max 6 SHO links
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Soft Handover Simulation Results II
the overhead due to simultaneous connections becomes higher!
0
0,5
1
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1 2 4 6
Max . Active Se t S ize
M
ean
Num
berofActive
Links
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Handover f1
f2 always neededbetween layers
Hierarchical cell structure (HCS)
Macro Micro Macro
f1
f2
f1
Handover f1
f2 neededsometimes at hot spot
Hot-spot
f1
f2f1 f1
Hot spot
Inter-Frequency Handover
Hard handover
Inter-frequency measurements of target cell needed in both scenarios
Mobile-assisted handover (MAHO)
slotted (compressed) mode for inter-frequency measurements to find suitabletarget cell
also supports GSM system measurements Database assisted handover (DAHO)
no measurements performed on other frequencies or systems
use cell mapping information stored in data base to identify the target cell
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Power Control: Basics
Controls the setting of the transmit power in order to:
Keep the QoS within the required limits, e.g. data rate, delay and BER
Minimise interference, i.e. the overall power consumption
Power control handles:
Path Loss (Near-Far-Problem), Shadowing (Log-Normal-Fading) andFast Fading (Rayleigh-, Ricean-Fading)
Environment (delay spread, UE speed, ) which implies differentperformance of the de-interleaver and decoder
Uplink: per mobile
Downlink: per physical channel
Three types of power control: Inner loop power control Outer loop power control (SIR-target adjustment)
Open loop power control (power allocation)
Downlink power overload control to protect amplifier Gain Clipping (GC) Aggregated Overload Control (AOC)
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Near-Far Problem: Spreading sequences are not orthogonal
(multi-user interference) Near mobile dominate Signal to interference ratio is lower for far
mobiles and performance degrades
The problem can be resolved throughdynamic power control to equalize allreceived power levels
AND/OR
By means of joint multi-user detection
Near-Far Problem Power Control
NodeB
UE 1
UE 2
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Closed Loop Power Control
Closed loop power control is used on channels, which are established in bothdirections, such as DCH
There are two parts Inner Loop Power Control (ILPC): receiver generates up/ down
commands to incrementally adjust the senders transmit power
Outer Loop Power Control (OLPC): readjusts the target settings of theILPC to cope with different fading performance
NodeB
UE
control command: Up/Down
Example: Uplink Closed Loop Power Control
Inner Loop(1500 Hz)
SIR > SIRtarget ?
Outer Loop( 100Hz)
target adjustmentBLERtarget
RNC
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Impact of Power Control
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power/fading[dB]
time [sec]
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Eb
/N0
[dB]
speed = 3 km/h
Example: UMTS Closed Loop Power Control in the slow fading channel
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Power Control Performance
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5.5
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0 20 40 60 80 100 120
Velocity [km/h]
RequiredULSIR
[dB]
PedA
VehA
SIR requirement strongly depends on the environment (due to differentfast fading conditions Jakes models)
outer loop power control needed to adapt SIR
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Open Loop Power Control
Open loop power control is used on channels that cannot apply closedloop power control, e.g. RACH, FACH
The transmitter power is determined on the basis of a path loss estimatefrom the received power measure of the opposite direction
To avoid excessive interference, probes with incremental power stepsuntil a response is obtained: power ramping
NodeB
UE
Open Loop Power Control on RACH
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CDMA Overload
CDMA systems tend tobecome unstable More traffic increases the
interference More interference requires
higher power More power increases the
interference Methods are required to limit
the system load Restrict the access to the
system Overcome overload
situations
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Interference in CDMA Networks
Frequency reuse factor is one
CDMA is subject to high multiple access interference
Soft capacity: CDMA capacity (e.g. number of users) determined by theinterference is soft
Handling of interference is the main challenge in designing CDMA networks
Interference Problem
Inter-Symbolinterferenz (ISI) Delayed components from thesame user signal interfere due tomultipath propagation
Multiple Acces Interference MAI Different user signals interferedependent on the access scheme
Intra-Cell Interference Interference caused by the users
belonging to same cell
Inter-Cell Interferenz Interference caused by the usersbelonging to neighbor cells.
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Cell Breathing
CDMA systems: cell size depends on the actual loading
Additional traffic will cause more interference
If the interference becomes too strong, users at the cell edge can nomore communicate with the basestation
CDMA interference management
Restriction of the users access necessary
Cell breathing makes network planning difficult
Example: cell brething with increasing traffic
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Cell Breathing (contd.)
coveragelow load
coveragemedium load
coveragehigh load
Coverage depending on load: load causes interference, which reduces thearea where a SIR sufficient for communication can be provided
yellow area: connection may drop or blocked
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Coverage vs. Capacity
Capacity depends on:
QoS of the users (data rate, error performance (bit-error-rate))
User behaviour (activity)
Interference (out of cell)
Number of carriers/ sectors
Coverage (service area) depends on:
Interference (intra- & inter-cell) + noise
Pathloss (propagation conditions)
QoS of the users (data rate, error performance (bit-error-rate))
Thus, trade-off between capacity and coverage
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Coverage vs. Capacity
Downlink limits capacity while uplink limits coverage Downlink depends more on the load (users share total transmit BS power)
0 20 40 60 80 100 120 140 160 180 2000
0.5
1
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3.5
Downlink
Uplink
Erlangs (2% GOS)
M
aximum
cellradius(km)
13kbps circuit switched service capacity versus maximum cell radius
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Example of Coverage and Best Server Map
Application: RF engineering (cell layout)
Legend: violet indicates high signal level, yellowindicates low level
coveragemap
Application: HO decision
Legend: color indicates cell with bestCPICH in area
bestse
rvermap
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Load Control: Basics
Main objective:
Avoid overload situations by controlling system load
Monitor and controls radio resources of users Call Admission Control (CAC)
Admit or deny new users, new radio access bearers or new radiolinks
Avoid overload situations, e.g. by means ofblocking the request
Decisions are based on interference and resource measurements Congestion Control (ConC)
Monitor, detect and handle overload situations with the alreadyconnected users
Bring the system back to a stable state, e.g. by means ofdropping an existing call
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Resource Consumption
Service/BLER-dependentresource consumption
Uplink example: Service I: Voice
Rb = 12.2kbps, Eb/Nt= 5dBI = 0.99%
Service II: Data
Rb = 144kbps, Eb/Nt= 3.1dBII = 7.11%
In downlink there is additionaldependency on the locationof the user Cell center low
consumption Cell edge high
consumption
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Admission/ Congestion Control
Basic algorithm Admission control is triggered
when load thr_CAC New users are blocked Existing users are not
affected as long as load